My cake instructor showed us how to do this last year and I love it. I do not tie a knot on the end, though. If you twist the bag and sorta "fold" the top end over the icing isn't going to come out anyway. It saves so much time on cleaning the bags (I only use the featherlight bags, I don't like the clear disposable ones). Plus, if you have to change colors when you need to use that same tip, that can easily be done, just pull the saran bag out and insert the new one. You will have to squeeze out the icing that is in the tip before you can use the new color, but it works great!
I fold mine different than that too. I put the frosting in the center of the wrap, fold the end (toward me) over to the opposite side of the frosting, sorta like engulfing it, then roll it. I then pick it up, twist it like you would twist a peppermint wrapper, "massage it" until one end is narrower than the other, then snip off the narrow end and insert into bag. That way the narrow end goes into the tip part easier, thus not really getting into the bag.
Edited to add this tip: It helps if you have a good clingy plastic wrap. If you don't have a good wrap, I can see where you would want to knot the top end. I don't like the Cling Wrap brand because it doesn't "stick" very well to anything. Sam's has a very good one in a black box - you can get it at Walmart. Kroger also has a good one. Both of those brands' boxes have the opening at the top of the box instead of the entire side coming up.
